KIEV - Ukraine's Chernobyl plant, scene of the world's worst civilian nuclear accident, will finally shut down Friday, to international plaudits, but the crippled site will remain a radioactive time bomb for decades to come.

Visiting dignitaries will gather from a dozen countries, including the United States, Germany, Russia and Japan, to deliver a requiem to the infamous plant whose eruption 14 years ago contaminated three quarters of Europe.

An estimated 15,000 to 30,000 people have died as a result of the April 26, 1986 explosion of reactor number four, which spewed radiation into the atmosphere equivalent to 500 times that of the atomic bomb dropped on Hiroshima in 1945.

Experts say some 160 tonnes of radioactive material remain inside the broken reactor, smothered by a hastily-erected concrete shield, known as the "sarcophagus," which is already cracking up.

Chernobyl's number two reactor was shut down in 1991 following a fire. Its number one reactor was taken out of service in 1996.

Now reactor number three, the last one still in service at Chernobyl, is due to close on December 15 after a two-week period in which the frailty of its Soviet-era design has been cruelly exposed by two unscheduled shutdowns.

An electric short-circuit on November 27 put a halt to the reactor's operations for four days, while the discovery of a radioactive leak in its aging cooler system again brought the reactor to a standstill on December 6.

But Chernobyl's decrepitude has all the more fears because its RBMK reactors -- the most powerful in terms of capacity and a source of pride when construction began in 1972 -- are also regarded as among the least stable.

Nevertheless the West is likely to heave a sigh of relief at Friday's shutdown as it comes after years of diplomatic wrangling between Ukraine and its international partners.

Under a 1995 protocol, the West promised 2.3 billion dollars, comprising 500 million in grants and 1.8 billion in loans, to help provide substitute power for Ukraine, which still derives about five percent of its electricity from Chernobyl.

In addition to making the site safe -- or, at any rate, safer -- the G7 funds will be spent on social programmes to help reduce the economic fallout of closure, and on boosting safety at Ukraine's four other nuclear centres (Rivne, Khmelnitsky, Pivdenno Ukrainska and Zaporizhiya).

Yet the celebrations Friday are bound to be somewhat muted, because the shutdown at Chernobyl represents only a partial victory, with the defunct reactors likely to pose a threat for decades to come.

International fears focus on the sarcophagus itself -- which helps to explain why the West is digging into its pocket for an extra 760 million dollars as part of an eight-year effort to forestall a catastrophe.

Already the roof of a structure that was hastily and hazardously installed between May and November 1986, in the immediate aftermath of the explosion, is beginning to crack.

Experts worry that the cracks are letting in rainfall, adding to an unknown quantity of water already inside the ruins, owing to firefighters' efforts 14 years ago to douse the blaze.

Another worry is that the concrete, weakened by ceaseless radioactive bombardment and the elements, could collapse, causing up to 160 tonnes of radioactive magma to be hurled into the atmosphere like volcanic dust.

One of the most daunting problems is a huge stockpile of low- to medium radioactive waste, comprising pieces of metal, concrete, plastic or wood, that have been recklessly dumped at a decrepit storage site on the complex.

However, defusing the nuclear time-bomb by removing the magma to a more secure site is not on anybody's agenda at the moment. The operation is considered too difficult, too dangerous and too expensive.

As a result, it is hard to think of Friday's shutdown as the end of the Chernobyl story: it will stop generating nuclear power on December 15, but it will continue -- as it has done since April 26, 1986 -- to generate nuclear fears for a long time to come.